It's about a man who gets admitted to the hospital for some illness (I think it was cancer) and in the same room is a very old woman and her grandson. One day he notices the grandson sucking this black blob out of the old woman's mouth, and afterwards she seems to feel better. Later, he sees a doctor try to force himself on the boy, and the black blob jumps between their mouths and makes the doctor really really sick. It turns out the boy is actually a monster that eats people's illnesses. He has a really strange way of talking, like some mix of kanji and hiragana, that the scanlation translated as alternating caps and lowercase letters. In the end, the boy asks the MC if he can eat his illness and stay with him.
If you know the title of this manga please let me know! I'm starting to think I imagined it
